In recent years, in consideration of environmental disruption of global scale such as destroying the ozone layer, global warming and acid rain, measures against environmental pollution of the earth has been advocated. At the same time, from the viewpoint of environmental protection, various control have been made by the governments of many countries. In such circumstances, releasing organic solvents in the air has become a big problem and, consequently the tendency to use no solvent has become remarkable in many industries. Also in the coating industry, powder coatings have been placed with many hopes to become substitutes of conventional solvent-type coatings. The coat film performance of such powder coatings, however, is not satisfactory comparing with that of the solvent-type coatings.
The powder coatings are advantageous over the solvent-type coatings, in that they are free from solvents and a thick coat film having a thickness of 30 to 500 .mu.m can be formed by one coating operation. On the other hand, they are problematic in that a coat film becomes thick, so that workability of the coat film deteriorates. Polyester-type powder coatings have been employed in the fields where weather resistance is required such as road materials and construction materials. In recent years, in view of the maintenance-free coatings, a polyester-type powder coatings with much higher weather resistance have been investigated. However, such highly weather-resistant polyester-type powder coatings also have a problem that workability of the coat film deteriorates. As coatings for precoated metal which are subjected to working such as bending, punching and embossing after coating, epoxy-type and polyester-type powder coatings have been widely investigated, and demand for development of powder coatings which can endure severe processing conditions after coating has been increased. Although all of the powder coatings of the above types have become to have coat film performances close to those of the solvent-type coatings, it is, in fact, hard to say that they have balanced toughness of a coat film, such as workability and impact resistance, performance of a coat film, such as appearance and weather resistance, and powder characteristics such as an anti-blocking property.
On the other hand, multilayer polymer particles represented by a core-shell polymer in which a core is composed of a rubbery polymer and a shell is composed of a glassy polymer have heretofore been known. Examples in which such a core-shell polymer is applied to a powder coating are disclosed in Japanese Examined Patent Publication Nos. 25709/1987 and 12926/1997 (corresponding to EP 0 750 023 A1).
The Japanese Examined Patent Publication No. 25709/1987 discloses a technique relating to an epoxy resin composition for a powder coating which is excellent in impact resistance, the composition comprising 3 to 35 parts by weight of a rubber obtained by graft polymerization of rubber particles containing not less than 50% of toluene-insoluble components with epoxy acrylate, a monoethylenic glycidyl ester and an ethylenic monomer. With respect to this technique, however, ring-opening and crosslinking reactions of an epoxy group easily take place during manufacture or storage of the particles due to high reactivity between glycidyl groups originated from the monoethylenic glycidyl ester. As a result, there is a problem that dispersibility to a powder coating and appearance of a coating film deteriorate.
The Japanese Examined Patent Publication No. 12926/1997 discloses a technique relating to a thermosetting powder coating composition excellent in impact resistance and chipping resistance, the composition comprising a (meth)acrylic copolymer having a glycidyl group, a multivalent carboxylic acid compound and rubber particles having a core with a glass transition temperature (Tg) of not over 20.degree. C. and a glycidyl group-free shell having a Tg of not less than 40.degree. C. In this technique, core-shell particles obtained without using either crosslinking monomers or grafting monomers during its manufacture and a commercially available impact modifier are used. Since, although an improving effect on impact resistance and chipping resistance can be observed, there is no satisfactory effect on improvement in workability of a coat film and an anti-blocking property of the powder coating may deteriorate, there is a problem that its application range is limited.
In consideration of the above problems, the object of the present invention is to provide a powder coating composition which can show excellent dispersibility to various powder coatings and which can improve workability and impact resistance of a coat film while maintaining original characteristics of a powder coating such as appearance of a coat film and an anti-blocking property.